This invention relates to a plasma ashing apparatus which has a cathode electrode and in which a resist film coated on a semiconductor substrate is removed by ashing by utilizing a plasma.
In order to fabricate fine integrated circuits, it has hitherto been practiced to provide on a surface of a semiconductor substrate a resist film having thereon a circuit pattern and then to etch this pattern into an insulating film, a semiconductor film, or a metallic film which lies under the resist film.
The resist film is removed from the substrate after the etching treatment has been finished. As a method of removing the resist film, there are a wet treatment method in which chemicals such as hydrogen peroxide, organic solvents or the like are used, and a dry treatment method in which the resist film is ashed by using an oxygen plasma.
In the wet treatment method, attention must be paid to the safety and, since impurities adhere to the substrate, this method is not suitable to fine fabrication of VLSI circuits.
In the dry treatment method, a resist film of C.sub.X H.sub.Y N.sub.Z coated on the substrate is removed by causing it to react with oxygen radicals generated in the oxygen plasma, thereby decomposing and evaporating it into CO.sub.2, NO.sub.2 and H.sub.2 O. Therefore, there is no problem, unlike in the wet treatment method, of safety and adhesion of impurities and is suitable to the fine fabrication of the substrate. In this case, the substrate and the resist film are heated for reaction with the radicals.
However, when the substrate coated with the resist film is subjected to irradiation of ion beams, the surface layer 2a of the resist film 2 is affected and hardened as shown in FIG. 1. As a consequence, stresses come to be accumulated inside the resist film 2 and, should the substrate 1 be heated suddenly, there are cases where the resist film 2 is exploded. Those flakes of the resist film 2 which are generated in this explosion stay on the substrate 1 or inside a vacuum treatment chamber as dust and residual matter, and become obstacles in the fine fabrication of the substrate 1. According to experiments, the lower limit temperature of explosion of the resist film 2 is from about 70.degree. C. to 160.degree. C., though it varies with the ion implantation conditions and the kind of resist film, and it has been found that the larger the width of the resist film 2, the easier the explosion.
In order to eliminate this kind of obstacle associated with the dry type of ashing method, the inventors of this application previously proposed an improved plasma ashing apparatus as shown in FIG. 2 (U.S. patent application Ser. No. 462,380). That plasma ashing apparatus is provided with a vacuum treatment chamber 5 to one side of which is connected an introduction pipe 3 for introducing a reactive gas such as oxygen or the like and, also, to a bottom of which is connected a vacuum exhaust pipe 4 which is in communication with an exhaust pump 4a. In an intermediate portion of the introduction pipe 3 which is connected to a reactive gas source 6, there is provided a plasma applicator 9 which comprises a microwave power source 7 and a microwave discharging member 8. Inside the vacuum treatment chamber 5, there are disposed a heating means 10 in the form of an electric heater to heat the substrate 1 and two pieces of plate-like electrodes which are parallel to each other, namely, a substrate electrode 11 and a counter electrode 12. By means of pins 13 which move up and down through the substrate electrode 11, a substrate 1 coated with a resist film is placed movably upward and downward between a position in which it contacts the surface of the substrate electrode 11 and a position in which it is lifted off the surface of the substrate electrode 11. The substrate electrode 11 is connected to an RF power source 14 and the counter electrode 12 is connected to ground 15. Numeral 16 denotes a lifting apparatus to move up and down the pins 13.
In case the resist film 2 which has been affected and hardened at its surface layer 2a as shown in FIG. 1 is removed by ashing in this ashing apparatus, a reactive gas such, for example, as oxygen gas is introduced through the gas introduction pipe 3. An RF plasma is generated between the substrate electrode 11 and the counter electrode 12 by supplying electricity from the RF power source 14 to the substrate electrode 11 on which the substrate 1 is placed. The resist film 2 is subjected to etching by the oxygen ions, for example, generated therein, thereby peeling off the affected and hardened portion of the surface layer 2a. Subsequently, the oxygen radicals are introduced into the vacuum treatment chamber 5 while the substrate 1 is heated by the heating means 10 and the plasma applicator 9 provided in the introduction pipe 3 is operated. The oxygen radicals are thus caused to react with the remaining portion of the resist film 2 on the substrate 1, thereby decomposing and evaporating it into CO.sub.2, NO.sub.2 and H.sub.2 O for exhausting out of the vacuum treatment chamber 5. The counter electrode 12 is provided with small perforations 17 through which the reactive gas can pass.
In case of peeling off the surface layer 2a of the resist film 2 by etching by ions with the ashing apparatus as shown in FIG. 2, the counter electrode 12 is in ground electric potential and the substrate electrode 11 becomes the negative electric potential. Ions in the plasma are, however, hard to be collected to the side of the substrate electrode 11, and this electrode 11 gives rise to self-bias voltage, resulting in incidence of high-energy ions on the substrate 1. As a consequence, there was a disadvantage in that the substrate 1 itself or the circuit formed on the substrate 1 is damaged.
This invention has an object of providing a dry type of plasma ashing apparatus in which the explosion of the resist film does not occur. Another object of this invention is to provide an ashing apparatus in which the resist film can be ashed without damaging the substrate.